1. Field of the Invention
The present invention relates to a liquid crystal display device, and particularly relates to a transflective liquid crystal display device capable of a display in a reflection mode utilizing external light and a display in a transmission mode in which backlight is turned on.
2. Description of the Related Art
Following the widespread use of cellular phones and personal digital assistants, portable information processing devices are on the market in volume. While reduction in size and weight is underway, high performance and color display are in demand in these portable information processing devices. In the display devices of these portable information processing devices, transflective liquid crystal display devices, which is capable of a reflection mode display by external light with less power consumption and a transmission mode display by backlight to improve visibility during night and the like, are mainly used.
In a conventional transflective liquid crystal display device, a polarizer and a transflective reflector are placed at a side opposite to a visible side of a liquid crystal cell. However, this transflective reflector uses a semitransparent plastic film or a thin metal film of half mirror, and its reflectivity R is expressed by
Reflectivity R=100xe2x88x92(transmittance T+light absorption ratio A) [%], 
and even if a material with a very low light absorption ratio A is used, the reflectivity R becomes low when the transmittance T is made high, and one of the transmission mode display and the reflection mode display becomes a dark display.
Thus, for example, as disclosed in JP, 2000-330107, A, a liquid crystal display device, which includes a scattering layer with a polarization maintaining property, a light guide film formed with a special projection and depression form on a top face thereof and a reflector on a side opposite to a visible side of a liquid crystal cell including a pair of polarizers, is proposed, and according to this, a bright reflection display is made possible though the transmittance is very high.
The conventional liquid crystal display device is constituted of an upper polarizer 110, a liquid crystal cell 100, a lower polarizer 120, a scattering layer 90, a transparent light guide film 30 and a light source 40, and a reflector 70, as shown in FIG. 13. Prismatic projections and depressions constituted by slant faces 30b each with an inclination angle of 35 to 40 degrees and flat faces 30a each with an inclination angle of 10 degrees or less are formed on a top face of the light guide film 30. The light guide film 30 and the light source 40 constitute a backlight.
In this liquid crystal device, when the light source 40 is turned on, incident light I1 to the light guide film 30 from the light source 40 is reflected downward by the slant face 30b and is reflected again by the reflector 70, it is transmitted through the flat face 30a, further transmitted though the scattering layer 90, the lower polarizer 120, the liquid crystal cell 100 and the upper polarizer 110 to be emitted light O1 and emitted to the visible side (the upper side in FIG. 13). Since no transflective films such as half mirrors are not provided in an emission route of the light from the above-described light source 40, in this liquid crystal display device, a bright display in the transmission mode can be obtained as in an ordinary transmission type liquid crystal display device.
Meanwhile, in the case of the reflection mode display by external light, incident light I2 from the visible side is transmitted through the upper polarizer 110, the liquid crystal cell 100, the lower polarizer 120 and the scattering layer 90, and reaches the light guide film 30. Further, the light which reaches the flat face 30a of the light guide film 30 travels in a straight line inside the light guide film 30, reflected by the reflector 70, emitted from the flat face 30a of the light guide film 30, transmitted through the scattering layer 90, the lower polarizer 120, the liquid crystal cell 100 and the upper polarizer 110, and emitted to the visible side as emitted light O2. In the case in which a mirror tuning reflector such as a thin metal film is used for the reflector 70, the area other than that in the specular reflection direction becomes dark, and therefore the scattering layer 90 is provided.
Here, if the scattering layer 90 has a high degree of depolarization, namely, if it has the property of changing the polarizing state of the incident light to a large extent, the incident light I2 and the reflected light which reaches the lower polarizer 120 have different polarizing states, and the amount of light transmitted through the lower polarizer 120, namely, the amount of light of the emitted light O2 decreases, thus making the display dark. However, by using the scattering layer 90 having a polarization maintaining property, a bright display in the reflection mode is made possible.
However, it is necessary to design the light guide film 30 for each liquid crystal cell because a pitch of the prismatic projections and depressions on the surface of the light guide film 30 and a pixel pitch of the liquid crystal cell 100 interfere with each other to cause moire, thus causing a disadvantage of increasing the cost.
If a prism sheet is placed between the light guide film 30 and the scattering layer 90 for the purpose of increasing transmission luminance, polarization of the light reflected by the reflector 70 is depolarized when it passes through the prism sheet in the case of the reflection mode, and a part of the reflected light is absorbed in the lower polarizer 120 and is not emitted to the visible side, thus causing the disadvantage that the display in the reflection mode becomes dark.
Next, another prior art by which the same effect can be obtained without using the light guide film in a complicated shape as in the above prior art will be explained. This is, for example, a liquid crystal display device disclosed in JP, 2001-83502, A, and a schematic configuration thereof is shown in FIG. 14. In FIG. 14, the parts corresponding to those in FIG. 13 are given the same reference numerals and symbols.
As a light guide film composing backlight with the light source 40 by LED, this liquid crystal display device comprises a transparent light guide film 50, which is formed to have fixed thickness by a transparent resin such as acrylic resin, polycarbonate resin, and amorphous polyolefin resin, an inorganic transparent material such as glass, or a composite of them, and which has a number of small projections (omitted in the drawing) formed on its surface.
It also comprises a reflection type polarizer 60 having a reflection axis and a transmission axis between the light guide film 50 and the scattering layer 90.
In this reflection type polarizer 60, the reflection axis and the transmission axis intersect each other orthogonally, and the transmission axis is placed to substantially correspond to a transmission axis of the lower polarizer 120, whereby a bright display in the reflection mode is obtained.
Namely, the incident light I2 from an upper side, part of which is depolarized when transmitted through the light guide film 50, is reflected by a reflector 70 in a state in which the polarization direction is rotated. Then, the reflected light of which polarization direction is not rotated is transmitted through the reflection type polarizer 60, the lower polarizer 120, the liquid crystal cell 100 and the upper polarizer 110 and emitted to the visible side as emitted light O21. Further, the light of which polarization direction is rotated inside the light guide film 50 is reflected downward by the reflection type polarizer 60, transmitted through the light guide film 50 again, reflected by the reflector 70 and reaches the reflection type polarizer 60 again. On this occasion, the polarization direction of a part of the light is rotated inside the light guide film 50 and aligned in a direction of the transmission axis of the reflection type polarizer 60, and thus the part of the light is transmitted through the reflection type polarizer 60 and emitted to the visible side as emitted light O22. As described above, by repeating reflection several times, a substantial part of the incident light I2 is emitted to the visible side, and a bright display in the reflection mode can be obtained.
On the other hand, in the transmission mode, incident light I1 onto the light guide film 50 from the light source 40 is reflected downward by the small projections (illustration is omitted) formed on the top face of the light guide film 50, the reflected light is reflected by the reflector 70 and transmitted through the light guide film 50, and reaches the reflection type polarizer 60. In this situation, the light polarized in the direction of the transmission axis of the reflection type polarizer 60 is transmitted and is emitted to the visible side as emitted light O11.
The residual light is reflected by the reflection type polarizer 60, transmitted through the light guide film again, reflected by the reflector 70, and reaches the reflection type polarizer 60 again. On this occasion, the polarization direction is rotated inside the light guide film 50, and therefore the light which comes to be polarized in the direction of the transmission axis of the reflection type polarizer 60 is transmitted through the reflection type of polarizer 60 and emitted to the visible side as emitted light O12. By repeating reflection several times as described above, a substantial part of the incident light I1 is emitted to the visible side, and a bright display can be also obtained in the transmission mode.
However, in the light guide film 50 of this prior art, the above-described small projections on its top face are formed by injection molding, and on this occasion, retardation unevenness occurs, which causes unevenness in depolarization ability. As a result, even if incident light is repeatedly reflected between the reflection type polarizer 60 and the reflector 70, the unevenness is not eliminated completely, and thus unevenness easily occurs to the brightness in display.
When the light guide film of only a transparent resin without the small projections being provided on its top face is used, a favorable bright display can be obtained in the reflection mode, but in the transmission mode, the light from the light source 40 is not favorably reflected upward, and thus the amount of emitted light to the visible side decreases, resulting in a dark display.
Further, either of the above-described two conventional liquid crystal display devices provides a display with a sense of depth because the reflector 70 is located with a distance from the liquid crystal cell 100 in either of the devices.
The invention is made to eliminate various problems in the conventional transflective liquid crystal display device as described above, and has its object to make it possible to obtain a bright display in both a reflection mode and a transmission mode, to eliminate occurrence of moire with a simple structure and eliminate occurrence of unevenness to the brightness of the display, and to obtain a display without a sense of depth in the reflection mode.
The liquid crystal display device according to the invention is constituted as follows to attain the above-described object.
A liquid crystal cell holding nematic liquid crystal with twisted alignment between a first substrate having a first electrode and a second substrate having a second electrode, an upper polarizer provided outside the second substrate at a visible side of the liquid crystal cell, a lower polarizer and a scattering layer provided outside the first substrate at a side opposite to the visible side of the liquid crystal cell, a light guide film of side light type provided outside the lower polarizer and light scattering layer, and a reflector provided outside the light guide film are included.
The light guide film is a light guide film with a light scattering property in which particles with the scattering property are mixed, and each of the scattering layer and light guide film with the light scattering property has a property of scattering incident light while substantially maintaining a polarizing state thereof.
Further, it is preferable to provide a reflection type polarizer having a reflection axis and a transmission axis between the light guide film and the lower polarizer or the scattering layer. Alternatively, instead of the reflection type polarizer, a prism sheet or a scattering sheet may be provided.
Furthermore, it may be suitable to provide the reflection type polarizer having the reflection axis and the transmission axis between the light guide film and lower polarizer or the scattering layer and further provide either of the prism sheet or the scattering sheet, or both of them between the light guide film and the reflection type polarizer.
Further, it is preferable to provide one or a plurality of optical compensators between the second substrate of the liquid crystal cell and the upper polarizer.
The reflector may be a reflection type polarizer.
It is preferable that the light guide film is constituted of a semitransparent material with a haze value indicating a scattering degree being 20% to 50%, and for example, it is preferable that it is constituted of a methacrylic resin in which fine particles of a silicon resin are mixed.
Further, if a color filter of a plurality of colors is provided at either one of the first substrate or the second substrate of the liquid crystal cell, a bright color display can be obtained in the reflection mode and the transmission mode.
The above and other objects, features and advantages of the invention will be apparent from the following detailed description which is to be read in conjunction with the accompanying drawings.